Wellbores (often referred to as well bores, bore holes, etc.) are formed in subterranean formations for various purposes including, for example, extraction of oil and gas from subterranean formations and extraction of geothermal heat from subterranean formations. Wellbores may be formed in subterranean formations using earth-boring tools such as, for example, drill bits (e.g., rotary drill bits, percussion bits, coring bits, etc.) for drilling wellbores and reamers for enlarging the diameters of previously drilled wellbores. Different types of drill bits are known in the art including, for example, fixed-cutter bits (which are often referred to in the art as “drag” bits), rolling-cutter bits (which are often referred to in the art as “rock” bits), and hybrid bits (which may include, for example, both fixed cutters and rolling cutters).
Fixed-cutter bits typically include a plurality of hard, durable cutting elements secured to a face region of a bit body for drilling through rock and other hard formations. These cutting elements may comprise polycrystalline diamond compact (PDC) diamond tables mounted to supporting substrates, free-standing thermally stable diamond products, or “TSPs,” natural diamonds, or diamond impregnated structures. Generally, PDC cutting elements of a fixed-cutter type drill bit have either a disk shape or a substantially cylindrical shape. A cutting surface comprising the hard, superabrasive material in the form of mutually bound particles of diamond, may be provided on a substantially circular end surface of each cutting element. To drill a wellbore with a drill bit, the drill bit is rotated and advanced into the subterranean formation. The drill bit may be placed in a bore hole such that the cutting elements are adjacent the earth formation to be drilled. As the drill bit rotates, the cutters or abrasive structures thereof cut, crush, shear, and/or abrade away (depending on the formation and the type of cutting elements employed) the formation material to form the wellbore. A diameter of the wellbore drilled by the drill bit may be defined by the cutting structures disposed at the largest outer diameter of the drill bit.
The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which may comprise a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface of the formation. It is also known to employ coiled tubing as a drill string. Often various tools and components, including the drill bit, may be coupled together at the distal end of the drill string at the bottom of the bore hole being drilled. This assembly of tools and components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the bore hole by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a down-hole motor, which is also coupled to the drill string and disposed proximate the bottom of the wellbore. The down-hole motor may comprise, for example, a hydraulic Moineau-type motor having a drive shaft, to which the drill bit is mounted, that may be caused to rotate by pumping fluid (e.g., drilling mud or fluid) from the earth's surface down through the center of the drill string and through the hydraulic motor to the drill bit, the drilling fluid being then flowing out from nozzles in the drill bit, and back up to the surface of the formation through the annulus between the outer surface of the drill string and the exposed surface of the formation defining the wall of the bore hole.
Reamers (also referred to in the art as “hole opening devices” or “hole openers”) may also be used conjunction with a drill bit as part of a bottom hole assembly when drilling a wellbore in a subterranean formation. In such a configuration, the drill bit operates as a “pilot” bit to form a pilot bore in the subterranean formation. As the drill bit and bottom hole assembly advance into the formation, the reamer device follows the drill bit through the pilot bore and enlarges the diameter of, or “reams,” the pilot bore.
As a bore hole is being drilled in a formation, weight and torque is applied to the drill string to turn the drill bit and any reamer employed therewith. The axial force or “weight” applied to the drill bit (and reamer, if used) to cause the drill bit to advance into the formation as the drill bit drills the bore hole is referred to in the art as the “weight-on-bit” (WOB).